1. Field of the Invention
The invention relates to a light emitting diode including a light emitting element and a phosphor layer. More particularly, the invention is concerned with a light emitting diode including a light emitting element and a phosphor layer in which the quantity of light and fluorescence attenuated by light absorption can be reduced and the efficiency of radiating the light to the outside of the light emitting diode can be enhanced.
In this specification, an LED chip per se is referred to as “light emitting element,” and the whole system including an LED chip-mounted package resin or lens system or other optical system is referred to as “light emitting diode” or “LED.”
2. Related Art
A light source comprising a blue light emitting element sealed with a phosphor-containing transparent resin by molding has hitherto been used as a white light source. In this white light source, the blue light emitting element emits blue light which is mixed with yellow-series light given off from the phosphor upon excitation with the blue light to provide white light. This conventional white light source, however, is disadvantageous in that light absorption by the blue light emitting element and light absorption by the phosphor layer are so large that white light takeout efficiency is low. Specifically, a part of the light emitted from the blue light emitting element is reflected from the phosphor and is again returned to the blue light emitting element. The inside of the blue light emitting element refracts light highly. Therefore, the returned light is less likely to go outside the blue light emitting element and thus is absorbed in its large proportion in the blue light emitting element. Further, light, which is to be passed through the phosphor layer and to be radiated to the outside of the light emitting diode, is partially absorbed in the phosphor layer and thus is attenuated before being externally radiated.
On the other hand, the inside of the phosphor layer in its portion adjacent to the blue light emitting element is irradiated with blue light, emitted from the blue light emitting element, at high irradiation density. Therefore, a large proportion of yellow-series light given off from the phosphor layer in its portion adjacent to the blue light emitting element upon excitation with the blue light emitted from the blue light emitting element is incident to the blue light emitting element. Here again, the yellow-series light incident to the light emitting element is less likely to go outside the blue light emitting element and thus is absorbed in its large proportion in the blue light emitting element. Further, the other part of the yellow-series light, which is to be passed through the phosphor layer and to be radiated to the outside of the light emitting diode, is partially absorbed in the phosphor layer and thus is attenuated before being externally radiated.
As is apparent from the foregoing description, according to the conventional light emitting diode, the blue light emitting element is provided in intimate contact with the phosphor layer. Because of this construction, a part of blue light emitted from the blue light emitting element is reflected from the phosphor and is returned to and is disadvantageously absorbed in its large proportion in the blue light emitting element. Likewise, a part of yellow-series light emitted from the phosphor upon excitation with the blue light emitted from the blue light emitting element is returned to and is disadvantageously absorbed in its large proportion in the blue light emitting element. Further, the blue light and yellow-series light, which are passed through the phosphor layer, are attenuated by absorption in the thick phosphor layer during the passage through the phosphor layer. Due to the occurrence of the above unfavorable phenomena, the efficiency of radiation of white light provided by mixing of the blue light with the yellow-series excited light to the outside of the light emitting diode is low.